Critically ill patients with shock requiring vasopressors are at a high risk of death. High output shock (also known as distributive shock) is the most common form of shock, and is often caused by sepsis [1]. When shock is treated with vasopressors, two main classes of vasopressors are in the intensivists' armamentarium: catecholamines and vasopressin type peptides [1]. Currently, no specific type of vasopressor (e.g. norepinephrine, vasopressin, dopamine) compared to another vasopressor has been shown to improve outcome [2]. All vasopressors have limitations and potential side effects. Patients treated with catecholamines for shock often develop tachyphylaxis thereby limiting the utility of these agents, and high doses of catecholamines can cause direct cardiotoxicity [3]. The toxic potential of catecholamines has been recently demonstrated in a randomized clinical trial of septic shock patients treated norepinephrine [4]. In this study, beta-blockade with esmolol was shown to improve survival in these patients by decreasing the heart rate. Thus, vasopressors that are not inotropes or chronotropes may be useful in patients with shock. One such vasopressor is vasopressin, which is most commonly used as an adjuvant with catecholamines. Vasopressin has been shown to improve outcomes in patients with less severe septic shock, but has toxicity (e.g. cardiac and mesenteric ischemia) at high doses and interacts with hydrocortisone [5]. In high-output shock, the patients are critically ill and mean arterial pressure cannot be maintained without vasopressors. High-output shock is defined as a cardiovascular Sequential Organ Function Assessment (SOFA) score of greater than or equal to 3 or 4 as well as a cardiac index of >2.4 liters/min/BSA 1.73 m2 [10]. In high-output shock, if blood pressure cannot be maintained, it is uniformly fatal. In patients that cannot maintain their blood pressure, the addition of a ‘rescue’ vasopressor in this setting could be useful.
A subset of patients with shock (including high-output shock and other types of shock) are catecholamine-resistant. That is, they are unresponsive (do not exhibit an appropriate increase in blood pressure) in response to treatment with a dose of a catecholamine equivalent to a dose of at least 0.2 mcg/kg/min of norepinephrine.
Angiotensin II (sometimes referred to herein as ATII) is a naturally occurring peptide hormone with endocrine, autocrine, paracrine, and intracrine hormonal effects. It is a potent direct vasoconstrictor, constricting both arteries and veins and increasing blood pressure [6]. It has a half-life in circulation of approximately 30 seconds, but while in tissue, its half-life may be as long as 15-30 minutes. ATII increases secretion of ADH and ACTH, and may potentiate sympathetic effects by direct action on postganglionic sympathetic fibers. It also acts on the adrenal cortex, causing it to release aldosterone [6,7]. High doses of angiotensin II have been reported to induce adverse side effects, including for example, mesenteric ischemia and bronchospasm.